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Verhandlungen der Internationalen Vereinigung fürTheoretische und Angewandte Limnologie



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nutrient limitation, oligotrophic lakes, sockeye salmon, Idaho

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"Phytoplankton production in lake ecosystems is frequently controlled by amounts of nitrogen and phosphorus (Smith 1982, Elser et al. 1990), as well as minor- and micro-nutrients (Wurtsbaugh & Horne 1983, Wurtsbaugh 1988, Lovstad & Bjorndalen 1990). Nutrient limitation is often studied to determine which nutrient(s) should be reduced to control lake eutrophication (Schindler 1974(. Conversely, nutrients have been used for decades to stimulate plankton and, subsequently, fish production in aquaculture (Bardach et al. 1972). More recently, lake fertilization has been used to augment salmon runs for commercial exploitation (Hyatt & Stockner 1985, lye et al. 1988), and it has been suggested for reversing losses in lake productivity due to decreases in anadromous salmon populations (Koenings & Burkett 1987). After returning to their natal lakes, adult salmon spawn, die, and decompose, often releasing significant amounts of marine-derived nutrients into the freshwater ecosystems (Juday et al. 1932, Kline et al. 1990). Koenings & Burkett (1987), for example, calculated that sockeye salmon carcasses contributed nearly 60% of the annual phosphorus loading to an Alaskan lake. Because phosphorus loading is often important in determining phytoplankton abundance in lakes (Vollenweider 1976), declines in returning salmon can lead to decreases in lake productivity, and lower production of the juvenile salmon that rear in the lake for 1-2 years. In the Sawtooth Valley of Idaho, USA, runs of the endangered Snake River sockeye salmon (Oncorhynchus nerka) to five rearing lakes in Idaho have declined more than 99% over the last century, primarily because of dams constructed along their migration route (Rieman et al. 1991, Bevan et al. 1994). Thousands of salmon once returned to these lakes (Bjornn et al. 1968). The decline of the Snake River sockeye has undoubtedly reduced nutrient loading and may have contributed to the current low fish production in the lakes. Although the primary threat to these salmon is mortality during downstream migration, it may be possible to help save this race of salmon by fertilizing the rearing lakes to elevate them a higher level of productivity. This remedy should increase the growth rates and abundances of wild and hatchery-produced sockeye salmon that will bin introduced into the lakes. Here we report on laboratory and field nutrient-addition bioassays to determine if lake fertilization could be used to stimulate phytoplankton production and aid in the recovery Snake River sockeye salmon."